For starters, hopefully you arent spamming heals with cleric stance on.. thats going to hurt you. Also, hopefully your tank has aggro under control so you dont have to spread heals. normally, you shouldnt run out of MP in those lower lvls (though it can happen). Like its been said above, later on youll get Shroud of Saints to help restore MP, and you will also get Divine Seal to boost your healing magic. Use those two together to recover the most MP.
But for now, endure it lol. Should be easier come lvl 30